Site Condition, Treatment Stage & VFD Considerations for Wastewater Pumps

Q. When dealing with wastewater treatment pumps, what concerns pertaining to site conditions are relevant? A. The type of pump that can be used depends on the location of installation. These location considerations include the size of the installation space, discharge piping alignments, wet-well depth, interior/exterior installation and power. The area where the pump is installed must allow space for proper mounting of the pump and driver assembly, for the inlet and discharge piping and valves, and for the necessary power wiring and switches. Operators and maintenance personnel must have enough space to access the pump and complete any required work or removal of the pump. Because their installation space is significantly different than that of other pumps, horizontal and vertical pump assemblies require special evaluation. New designs allow designers to create the space required to meet the new pump requirements. Retrofit designs may not afford this ability. The existing structure, wet well, hydraulics, control sequencing or installation requirements may become the primary considerations when end users are choosing a pump. The fluid being pumped can create specific requirements for the driver and/or the power distribution components that serve the pump assembly. These requirements can change the sizing of the components, so users should also evaluate these factors when installing the pump assembly. For more information on wastewater treatment pumps, see HI's newest guidebook Wastewater Treatment Plant Pumps: Guideline for Selection, Application, and Operation. Q. What types of pumps are recommended for the primary treatment stage in a wastewater facility?

A. The primary treatment step physically separates insoluble solids from floatable materials contained in the wastewater. For example, primary sludge consists of the solids generated from the primary sedimentation process. The primary sludge may be transferred to a solids-processing system, such as a gravity sludge thickener, where the solids are concentrated and pumped to the solids-handling facilities. The primary sludge stage can incorporate many types of pumps, including horizontal and vertical solids-handling rotodynamic, submersible solids-handling rotodynamic, piston, progressive cavity (PC) and rotary lobe. When incorporating a solids-handling rotodynamic pump (see Figure 4.3), end users must consider the change in consistency because the most concentrated sludge is pumped in the beginning of a pumping cycle. When most of the sludge has been pumped, diluted sludge with consistency and hydraulic characteristics similar to water will be pumped. To determine the required motor power for a rotodynamic pump, users should plot system curves and pump curves for both the most dense sludge and water. A PC pump (see Figure 4.18) can be used in the primary treatment stage and will deliver constant flow regardless of pressure. The flow is controlled by the pump speed. For low-solids contents, full speed is typically allowed. If sand and grit are in the sludge, the speed should be adjusted down to reduce wear. Because of their design with a metallic rotor in direct contact with a stationary elastomeric stator, PC pumps must not run dry. If using a PC pump, install a pressure guard that shuts the pump off when it experiences a pressure drop indicating dry run. For more information on wastewater treatment pumps, see HI's newest guidebook Wastewater Treatment Plant Pumps: Guideline for Selection, Application, and Operation. Q. What benefits result from implementing variable frequency drives (VFDs) in a wastewater treatment plant? A. VFD units are used to vary motor speed, which allows the pump to provide different rates of flow to meet system requirements. They also provide an option to reduce inrush current. They have time-adjustable ramp-up and ramp-down features to control motor acceleration and deceleration times. The controller may be programmed to electrically disconnect the motor on signal shut-down. VFDs can provide numerous functions in wastewater pump applications, which include:

• Regulate the operation of pumps or the station in relation to wet-well depth, discharge flow or force main pressure, which can potentially eliminate the need for control valves
• Provide energy-efficient operation of pumps by matching best efficiency operating points and/or matching pump operation to varying demand loads
• Ability to over-speed pumps to meet certain specific short-term application conditions
• Provide power phase conversion (producing three-phase power from a single-phase power supply)
• Create power at a different frequency (e.g., producing 60-Hertz [Hz] power when only 50 Hz is available)
• Adjust the pump operational speed (or torque) instead of using a control valve to restrict pump output, which reduces downstream flow or pressure
• Reduce mechanical stress of conditions such as cavitation and water hammer on both the pump and pump system

To minimize the risk of clogging the pump impeller, users should consider alternating pumps to create back-flush and to bring them to full speed before stopping or at the start of a pumping cycle. Operating pumps at low speeds or with slow increases and decreases in speed can cause pump clogging. For more information on wastewater treatment pumps, see HI's newest guidebook Wastewater Treatment Plant Pumps: Guideline for Selection, Application, and Operation.